Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. Gaseous fuels such as natural gas are emerging as appealing alternative fuels. For automotive applications, natural gas may be compressed and stored as a gas in cylinders at high pressure. A pressure regulator may then be used to supply the compressed natural gas (CNG) at lower pressures to an engine's intake port or engine combustion chamber. The pressure regulator may provide this gaseous fuel at a fixed, constant pressure to the engine, or it may be a variable pressure regulator which can provide gaseous fuel at varying pressures to the engine.
Fixed pressure regulators are commonly used in gaseous-fueled vehicles, but have several disadvantages. Supplying fuel at a fixed, constant pressure limits the dynamic range of the associated fuel injector. As such, even at low fuel demands, the quantity of fuel injected remains the same. This causes the fuel injector to be exposed to high pressures throughout operation, affecting the durability of the injector. In contrast, by using a variable pressure regulator, the dynamic range (or turn down ratio) of the injector can be increased. Providing a lower fuel pressure during low fuel demand allows for a longer fuel injection pulse width, which in turn allows for injections of lower, repeatable mass. Further, varying the pressure of gaseous fuel enables use of a lower pressure of gaseous fuel during engine cold start when only a marginal voltage may be available to open the injectors. Additionally, varying the pressure may allow rare, peak fuel demands to be satisfied without having to subject the injector to the durability challenge of injecting high pressure gaseous fuel at all times.
Despite these potential advantages, known variable pressure regulators are costly and prone to instability. For example, in some systems, variable pressure regulation is achieved by exposing the reference chamber of the regulator to intake manifold pressure. However, this dependency on intake manifold pressure limits the operability of the pressure regulator when intake manifold vacuum is not within a certain range. In other systems, pressure variability is achieved by changing the reference pressure via a valve from the high pressure source. As another example, one known variable pressure regulator varies the pressure of gaseous fuel by duty cycling a main valve between the regulator and the fuel rail. However, systems which rely on a single valve to perform pressure regulation, where the valve is subject to a flow of high pressure gaseous fuel, may not be adequately robust. Further, such systems require electronic pressure feedback to actuate the valve.
In one example some of the above issues may be addressed by a system for a gaseous-fueled engine, comprising: a gaseous fuel tank, a first pressure regulator comprising: a high pressure chamber coupled to the gaseous fuel tank, a low pressure chamber coupled to an engine fuel rail, and a reference chamber; a second pressure regulator coupled to the reference chamber; a first valve controlling fuel flow from the gaseous fuel tank to the engine fuel rail; a second valve controlling fuel flow from the second pressure regulator to the reference chamber; and a controller including memory with instructions therein for adjusting the valves based on a desired fuel rail pressure. In this way, the first valve may be actuated to supply gaseous fuel to the engine fuel rail at a pressure determined by the properties of the first pressure regulator. Then, in response to an increase in desired fuel rail pressure, the second valve may be actuated. The actuation of the second valve allows gaseous fuel to flow through the second pressure regulator into the reference chamber, thereby increasing the pressure of the reference chamber and low pressure chamber at a pressure determined by the properties of the second pressure regulator. Thus, the system enables at least two different fuel rail pressures such that peak fuel demand can be satisfied without maintaining peak fuel rail pressure at all times.
In another example, some of the above issues may be addressed by a method for regulating gaseous fuel pressure in an engine, comprising: increasing a regulating pressure in a low pressure chamber of a first pressure regulator by controllably increasing the flow of gaseous fuel into a reference chamber of the first pressure regulator via a second pressure regulator. In this way, multiple different fuel rail pressures may be set by altering the pressure in the reference chamber of the first pressure regulator. The flow of gaseous fuel may be controlled by valves in response to changes in desired fuel rail pressure without relying on mechanical pressure feedback to actuate or de-actuate the valves.
In still another example, some of the above issues may be addressed by a method for an engine cold start, comprising: closing a valve coupling a gaseous fuel tank to a high pressure chamber of a first gaseous fuel regulator and opening a valve coupling a fuel rail to a reference chamber of the first gaseous fuel regulator via a second gaseous fuel regulator. In this way, the gaseous fuel already present in the fuel rail may be used to establish a low injection pressure that may be used when marginal voltage is available to open the fuel injectors, as a lower injection pressure requires a lower injector opening voltage.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.